Definitely Clojure, both server and browser (clojurescript), and also using React from Clojurescript (Om). These in combination gave me a big new glimpse into t

Definitely Clojure, both server and browser (clojurescript), and also using React from Clojurescript (Om). These in combination gave me a big new glimpse into the wide variety of expressivity among languages, and how UI development can be significantly streamlined. I don't use Clojure as much any more but everything I learned has had a lasting impact and much of it trickles into my other work now, in other languages (mainly C++).